Wrapping wire around a cylindrical vessel or pipe has long been used both as a means of reinforcement and as a means of prestressing the wrapped structure. It is well known, therefore, that when a cylindrical vessel or pipe is subjected to an internal pressure, the circumferential shell stresses are exactly twice the longitudinal shell stresses. A tightly wrapped circumferential wire therefore will share the circumferential load to reduce the circumferential shell stresses by an amount equal to the tensile stresses in the wire. Hence, the pressure holding capacity of a given cylindrical vessel or pipe can be doubled by suitable wire wrapping without endangering the safety of the structure in the longitudinal direction.
Besides offering an efficient approach to essentially doubling the pressure rating of a given vessel or pipe, wire wrapping also offers significant advantages with respect to resisting catastrophic fracture. That is to say, once a failure has occurred, as by piercing, the wire wrapping will serve to minimize displacement and strain in the shell wall to minimize crack propagation and complete bursting of the structure.
Although many wire wrapping techniques and designs have been used and proposed for both vessels and line pipe, anchoring the wire to the shell has always been somewhat of a problem. Typically, the wire is anchored in such a manner that the end thereof is held mechanically to a bracket or some metallic anchoring structure which is welded, bolted or somehow affixed to the shell. All such anchors have the common attribute of presenting some protrusion of metal above the general surface of the wire wrapping. In such structures as prestressed concrete pipe, such protrusions are of little consequence because the wires and the protrusions are subsequently covered with a gunite cement coating. In the case of line pipe and pressure vessels, however, the protrusions, whether large or small, can be troublesome in that they may be jarred and damaged during handling. Although it has been proposed that such protrusions can be eliminated by welding the wire ends to the vessel or pipe shell, such a solution has not been deemed practical because the high tensile strength wire does not lend itself to welding. That is to say, to optimize the advantages of wire wrapping, it is of course desired that hard-drawn high tensile strength wire be used. If this wire is welded, the resulting heat will effectively temper the wire to substantially reduce its tensile strength. In addition, the current trend, particularly for low temperature applications, is to utilize line pipe and vessel shells having an increased tensile strength due to suitable heat treatment. Welding a wire thereto would also cause a localized weakening of the shell in the heat affected zone. In welding the wire to the shell, therefore, one may lose more than he gains by wire wrapping.